Traveling wave tube



Oct. 23, 1962 J. s. cooK TRAVELING WAVE TUBE Filed April 8, 1960l/vl/EA/TOR B S. COOK ATTOEV United States Patent O 3,060,341 TRAVELINGWAVE TUBE John S. Cook, New Providence, NJ., assigner to Bell TelephoneLaboratories, Incorporated, New York, NX., a corporation of New YorkFiled Apr. 8, 1960, Ser. No. 20,964 12 Claims. (Cl. S15-3.6)

This invention relates to traveling wave tubes and more particularly tomeans for producing low noise amplification in traveling wave tubes.

Velocity modulation devices such as the traveling wave tube have provencapable of amplification with reasonably high efficiency and stabilityover a very wide band of frequencies. Detracting from the significantadvantages realized by such devices, however, is the noise resultingfrom the utiliz-ation of the electron beam. In the conventionaltraveling wave tube, 6 decibels is the theoretical minimum noise figureVas is established in an article entitled The Minimum Noise Figure ofMicrowave Beam Amplifiers, by H. A. Haus and F. N. H. Robinson,Proceedings of the lInstitute of Radio Engineers, volume 43, pages981-991, August 1955. Further discussion as to how this minimum noisefigure may be reduced, and indeed, be made to approach zero, requires abrief discussion of the nature of an electron beam.

The conventional traveling wave tube achieves electromagnetic signalwave amplification through space charge wave modulation of an electronbeam. Any space charge wave which inherently exists on an electron beam,or is introduced onto the beam from some outside source, may propagatealong the beam at either of at least two phase velocities. It can beshown that the faster of these two phase velocities at any givenfrequency is higher than the mean, or D.C., velocity of the unmodulatedbeam, whereas the slower phase velocity is lower than the beams D.-C.velocity. The phase velocities which represent space charge wavepropagation `at a velocity higher Vthan D.-C. velocity will be referredto as the fact space charge mode, while those phase velocities whichrepresent wave propagation ata velocity lower than the D.-C. velocitywill be referred to as the slow space charge Inode. Anothercharacteristic of the beam is its dispersion. `It can be shown that inthe slow space charge mode the phase velocities of space charge wavesvary directly with frequency. In the fast mode, however, space chargewave velocities vary inversely with frequency.

A conventional traveling Wave tube effects amplification throughelectromagnetic signal wave interaction with the slow space charge modeof an electron beam. As is `Nell known, the unique characteristics ofthe slow space charge mode which permits wave amplification isdisadvantageous in that spurious noise power which is inherent on theslow mode of the beam cannot be extracted by ordinary methods. This isdue to the equally well-known fact that power transmitted in the slowspace charge mode is negative with respect to the unmodulated D.C. powerof the beam. lIn the conventional traveling wave tube one is thereforegenerally limited to methods of reducing noise power in the electron gunregion. Although sophisticated electron guns have been built whichproduce beams having a noise figure of less than 6 decibels, thesemethods of reducing noise are limited.

In the patent of C. F. Quate, 2,974,252, granted March 7, 1961, there isdisclosed a completely different approach to the problem of reducingnoise in a beam device. By making use of the principles of parametricamplification, the Quate device effects interaction between -a signalwave and the beams fast space charge mode, thereby achieving desiredamplification of the signal. Because fast mode ynoise power adds to theD.C. beam power, thus produc- "ice ing an increase in the absolute beamenergy, it can be extracted from the beam through `any of a number ofwell-known devices.

In the Quate device, energy for signal wave amplification is derivedfrom a source of pump energy which is at a higher frequency than thesignal energy. This mechanism for amplification is disadvantageous bycomparison to the conventional traveling wave tube which effectsamplification through the conversion of D.-C. beam energy to signal waveenergy. In the Quate device the electron beam merely serves as anon-linear transmission medium rather than as `an energy source.Further, the high frequency energy power required for the pump wave isoften very difficult to produce, particularly when power requirementsare high. Since high gain in the Quate device requires high pump power,one can see that highfrequency high-power operation of the Quate devicemay impose serious difficulties.

lt is an object of this invention to eliminate the effects of noisepower existing on an electron beam of a traveling wave tube.

It is ya specific object of this invention to eliminate the eEects ofnoise power loriginating on the slow mode of an electron beam of atraveling wave tube.

It is another object of this invention to reduce pump power requirementsfor very low noise electromagnetic wave amplification in a travelingwave tube.

These and other objects of the present invention are obtained in oneillustrative embodiment thereof which comprises an electron dischargedevice having an evacuated envelope with -an electron gun therein forforming and projecting an electron beam along an extended path. A slowwave circuit such as a helix is positioned along the path of iiow forpropagating signal energy in coupling relationship with the slow spacecharge mode of the beam. As 4the signal wave propagates along the slowWave circuit, longitudinal electric fields that are associated therewithinteract with the beam in a conventional wellyknown manner to produceamplification of the signal energy. Since interaction takes placeentirely in the slow mode, energy for amplification is derived entirelyfrom the D.-C. kinetic energy of the beam.

It is -a feature of this invention that pump energy be coupled to theybeam along a certain predetermined distance by means of a slow wavecoupler that is positioned between the electron gun and the slow w-aveinteraction circuit. It is a corollary feature of this invention thatthe coupled pump wave propagates at a velocity substantially equal tothat of the slow signal mode velocity plus one-half the difference invelocities of the slow signal mode and the fast idler mode. The termslow signal mode velocity is intended to denote the velocity of anuncoupled space-charge wave of the signal frequency propagating in theslow lmode of the beam; likewise, fast idler rnode velocity refers tothe velocity of an `uncoupled space-charge wave of the idler frequencypropagating in the fast mode of the beam. The idler frequency is equalto the difference of the pump and signal frequencies. Under theseconditions, -a beating phenomenon occurs Wh-ich causes signal frequencyslow mode noise to be transferred to the fast mode, and idler frequencyfast mode noise to be transferred to the slow mode. I

It is another feature of this invention that the slow wave circuit pumpcoupler have a length substantially equal to the square root of theproduct of the reduced plasma wavelength at the pump frequency andthereduced plasma wavelength at the idler frequency, that quantity dividedby the fraction of beam current modulation of the pump wave. Theaforementioned transfer Vbetween the fast and slow modes takes placeover a specific predetermined distance. When this transfer is complete,a re-transfer will take place if pump energy is still coupled to thebeam. The pump coupler is therefore of a predetermined length so thatpump energy is advantageously removed from the beam at a particulardistance that represents one complete cycle of energy transfer betweenthe fast and slow modes. At this point, fast mode idler frequency noiseexists substantially completely in the slow mode while slow mode signalfrequency noise travels in the fast mode.

It is a feature of one embodiment of this invention that fast mode noisestripping apparatus be included between the electron gun and the slowwave pump coupler. The stripping apparatus is constructed such that itwill extract beam noise from the fast idler mode so that the noiseenergy transferred to the slow mode is negligible and substantiallynoiseless interaction can be eiected.

It is a feature of another embodiment of this invention that the pumpfrequency be much higher .than the signal frequency. As will be shownhereinafter, the noise power transferred from the fast idler mode to thesignal slow mode is directly proportional to the ratio of the signalfrequency to lthe idler frequency. By making the pump frequency muchhigher than the signal frequency, this ratio ybecomes very small so thatfast idler mode noise power transferred to the slow mode is likewisevery small.

These and other objects and features of my invention will be more easilyunderstood with a consideration of the following detailed description,taken in conjunction with the accompanying drawing in which:

FIG. l is a schematic illustration of one embodiment of this invention;and

FIG. 2 illustrates .the phase velocities of certain waves which mayexist respectively on the slow wave pump coupler and the electron beamof the device of FIG. l.

Referring now to the drawing, the embodiment shown in FIG. 1 comprises atraveling wave tube 10 having an electron gun 12 and a collector 13 atopposite ends of an evacuated envelope 11. For purposes of illustration,electron gun 12 is shown as comprising a cathode 14, a beam formingelectrode 15, and an accelerating anode 16 which coact to form andproject an electron beam, schematically shown as 18, toward thecollector 13. Battery 20 maintains the various electrodes at properpotentials as is well known in the art. Suitable means for focusing theelectron beam are used which, because they are well known in the art,have not been shown.

Extending along a major length of the tube is a slow wave interactioncircuit 23. A signal source 24 is coupled to the input of circuit 23while a suitable load 26 is coupled to the output. Circuit 23 is of thegeneral type used in conventional traveling wave tubes; it delays theaxial phase velocity of the signal wave to approximately the velocity ofthe electron beam 18. More specically, it delays the signal wavevelocity to a value slightly below that of the D.-C. -beam velocity sothat longitudinal field components of the signal wave will be inapproximate synchronism with slow mode space-charge waves ofcorresponding frequency in beam 18. Interaction between the signal waveand the slow mode of the beam takes place in a well-known manner toproduce amplification of the signal wave.

Between ythe electron gun and the interaction circuit is a slow wavecoupler 27 for propagating pump wave energy from a pump source 28 incoupling relationship Awith beam 18. Directional coupler 30 channelspump energy from source 28 and transmission line 31 to transmission line32.

Interposed between electron gun 12 and slow wave coupler 27 is a noiseextraction helix 34 for removing fast mode idler frequency noise energyfrom the beam. The term idler frequency is used herein to denote thedifference in frequency of the pump and signal waves. Although the termsidler frequency and pump frequency are usually used in connection withparametric amplification devices, the present device is not to beregarded as a parametric amplifier; amplification is attained byconventional traveling wave tube techniques as pointed out hereinabove.After extraction, fast mode idler noise energy is transmitted to, anddissipated by, an impedance 35. It should be pointed out that elements23, 27 and 34 have been shown as helices only for purposes ofillustration; various other structures could also be used for couplingwave energy to and from the beam as is well known in the art.

The usefulness of noise extraction helix 34 and pump wave coupler 27 inconjunction with a conventional traveling wave tube can be appreciatedby a consideration of FIG. 2. Graph 37 illustrates the spectrum of phasevelocities of space-charge waves which may propagate along beam 18 whilegraph 33 illustrates a similar spectrum with reference to pump wavecoupler 27. Both graphs are one-dimensional, of the same scale, and showincreases in phase velocity from left to right as indicated by the arrowlabeled velocity The D.C. Velocity uo of beam 18 is used as a referencefor both graphs because all fast mode space-charge waves travel fasterthan uo, while all slow mode space-charge waves travel slower than uo.

The uncoupled velocity of a slow mode space-charge wave of the signalfrequency is shown on graph 37 by the position of ss, while the fastsignal mode velocity is shown by sf. Likewise, is is the slow idler modevelocity, while if is the fast idler mode velocity. The phase velocityof an uncoupled pump wave on coupler 27 is shown on graph 38 by theposition of p.

As is pointed out in chapter VIII of the book Traveling Wave Tubes, byI. R. Pierce, Van Nostrand Company, Inc., 1950, a wave of a givenfrequency that results from coupling between a slow wave circuit and anelectron beam may travel at either or a combination of three differentvelocities, which Pierce designated y1, y2 and ya. The relativevelocities of these three normal modes are shown on FIG. 2 as extendingthrough both graphs 37 and 38 because they represent propagation of acoupled wave which travels on both beam 18 and coupler 27 The power ofthe pump wave which is propagated along coupler 27 is quite small. Itcan ltherefore be shown that, for purposes of this discussion, possiblepump wave propagation at velocities y1 and ya can be neglected. This isprimarily due to the fact that the uncoupled circuit pump mode velocityp is `fairly widely separated, in terms of velocity, from y1 and y3, andtherefore a fairly high power pump wave is required to excite thesenormal modes to :any substantial extent. Coupler 27 is constructed suchthat the difference in velocities of y2 and ss is substantially equal tothe difference in velocities of y2 and if; the purpose of thisparticular condition will be explained hereinafter.

As is well known in the parametric amplifier art, the presence of a pumpwave on an electron beam tends to cause coupling between signalfrequency space-charge wave energy and idler yfrequency spacecharge waveenergy. When any two waves couple, the strength of coupling variesinversely with the difference in velocity of the waves. It can be shown,however, that the signal wave ss sees the idler wave if as if it wastraveling at its image velocity ifi, while the idler wave sees thesignal wave ss as if it was traveling at its image velocity sst. Hence,strong coupling between the fast idler wave and the slow signal waveoccurs at velocity ss, if, and if, ssi, and the coupled wave may travelIat either of these two velocities. From this standpoint, the slow andfast mode of the beam can be considered as being analogous to twocoupled transmission lines. The wave energy on the fast and slow modeswill beat together such that the coupled slow mode energy will begradually transferred to the fast mode while the coupled fast modeenergy is gradually being transferred to the slow mode. When thistransfer is complete the process repeats itself; transfers andre-transfers take place as long as coupled pump energy at velocity y2exists on the beam. It should be noted in passing that the uncoupledslow mode and fast mode velocities are imaged about the beam velocity unthat is, s is the same distance from uo as if; ss is the same distancefrom un as sf. Further, if the pump frequency is twice the signalfrequency, the signal and idler frequencies are equal. Therefore, if thepump frequency is twice the signal frequency, y2 should be at the samevelocity as uo.

The -purpose of coupler 27 is to transfer slow mode noise to the fastmode. Coupler 27 is therefore terminated a predetermined distance atwhich this transfer is complete. `It can be shown that the distancerequired for one complete cycle of energy transfer between the fast andslow modes, and therefore, the desired length L of coupler 27 is givenby:

Where Aql is the reduced plasma wavelength of beam 18 at the siqnalfrequency, M12 is the reduced plasma wavelength of the beam at 4theidler frequency, and .M is the fraction of beam current modulation bythe pump frequency (M=l being 100% beam current modulation). The reducedplasma Wavelength of a beam at various frequencies is readily calculableby methods Iwell known in the art.

It can be appreciated that upon leaving coupler 27, beam noiseoriginating in the slow signal mode has been transferred to the fastmode; the only noise existing at velocity ss is that which yformerlytraveled a-t a fast idler mode velocity if. Since .the fast idler modenoise has been stripped by extraction helix 34, the slow signal modecontains substantially no noise energy as it enters the interactionregion defined :by slow Wave helix 23. Conventional slow waveamplification can thereby be attained with substantially no noiseappearing with :the signal wave at the putput end of helix 23.

It is known that lthe effect of lf-ast idler mode noise in a parametricamplifier can be reduced through lthe use of a high frequency pump wave.It can be shown that the effective reduction of idler noise isdetermined by:

where P1 is the noise power originating in the fast idler mode, ws isthe signal frequency, wi is theA idler frequency, wp is the pumpfrequency and Pi is .the noise power appearing at the signal frequencysubsequent to parametric mixing, as a result of the fast idler noiseinput.

This relationship can be used in the present device to obviate thenecessity of ,fast idler mode noise extraction apparatus such as helix34. If, for example, the pump frequency is dive times the signalfrequency, from Equation 2, the fast idler noise power :transferred tothe slow signal mode lwill be equal to only one-fourth the noise poweroriginating in the fast idler mode. Of course, any other' desired ratioof pump frequency to signal frequency could be used depending upon thedesired degree of noise suppression. This method of noise reduction,however, has the `obvious disadvantage of requiring a high pumpfrequency. Consequently, for very high frequency operation it willusually he Adesirable to use fast idler mode noise stripping apparatus.

It is to ibe understood that the above-described embodiments areintended only for purposes `of illustration. Numerous other arrangementsmay .be devised by those skilled in the art without departing from thespirit and scope of my invention.

What is claimed is:

l. A `traveling wave tube comprising an electron gun for forming `andprojecting an electron beam, said beam being characterized by fast andslow modes of propagation, spurious noise energy, and a mean velocity,means :for propagating signal frequency energy in an interactingrelationship with the slow mode of said beam, means included betweensaid electron gun and said propagating means rfor extracting noiseenergy from .the fast mode of said beam, means included between saidextracting means and said propagating means for transmitting a wave ofapproximately Itwice said signal frequency in coupling realtionship withsaid beam, said transmitting means being so consttructed that thecoupled phase velocity of said wave is substantially equal to said meanbeam velocity, and means for collecting said beam.

2. A traveling Wave tube comprising an electron gun for forming andprojecting an electron beam along a path, said beam being characterizedby fast and slow modes of propagation and noise energy thereon, a sourceof signal frequency energy, a signal input line, a signal output line,means for amplifying said signal energy comprising slow wave circuitmeans extending between said input and output lines for propagating saidsignal frequency energy in coupling relationship with the slow mode ofsaid beam, and means for minimizing the noise content of the amplifiedsignal energy comprising means for transferring noise energy at saidsignal frequency from said slow mode to said fast mode.

y3. The traveling wave tube vof claim 2 wherein said transferring meanscomprises a source of pump energy and transmitting means interposedbetween said electron gun and said signal input line for propagatingsaid pump energy in coupling relationship with the beam.

4. The traveling wave tube of claim 3 wherein said transferring meansfurther comprises means for transferring fast mode noise energy at saidsignal frequency to said slow mode, and means included between saidelectron gun and said transferring means for extracting fast mode signal`energy from said beam whereby the fast mode energy that is transferredto the slow mode is substantially negligible.

5. An electron discharge device comprising a source of signal frequencyenergy, a source of pump frequency energy, means for forming andprojecting an electron beam, said beam being characterized by a slowsignal mode velocity and a fast signal mode velocity at which uncoupledsignal frequency energy may propagate and a slow idler mode velocity anda fast idler mode velocity at which uncoupled wave energy at a frequencyequal to the difference of said pump and signal frequencies maypropagate, means connected to said signal source for transmitting signalenergy in coupling relationship with the slow signal mode of said beam,and means for propagating pump energy in coupling relationship with saidbeam at a velocity intermediate said slow signal mode velocity and saidfast idler mode velocity comprising a slow Wave circuit interposedbetween said electron gun and said transmitting means which is connectedto said pump source.

6. The electron discharge device of claim 5 wherein said slow wavecircuit is so constructed that the difference of said coupled pump wavevelocity and said slow signal mode velocity is substantially equal tothe difference of said fast idler mode velocity and said coupled pumpwave velocity.

7. The electron discharge device of claim 5 wherein said electron beamis characterized by a first reduced plasma wavelength at said signalfrequency and a second reduced plasma wavelength at a frequency equal tothe dilerence of said pump and signal frequencies, and wherein said slowwave circuit is of a length L substantially determined by:

where 1 is said rst reduced plasma wavelength, )t2 is said secondreduced plasma wavelength, and M is the fraction of beam currentmodulation of said pump wave.

8. The electron discharge device of claim 5 wherein 7 said pumpfrequency is at least fourV times said signal frequency.

9. The electron discharge device of claim` 5 further comprising meansincluded between said electron gun and said slow wave circuit forextracting wave energy from said beam which travels at said fast idlermode velocity.

10. A traveling wave tube comprising an electron gun for forming andprojecting a beam of electrons having fast and slow modes of propagationand noise waves thereon, means for propagating signal frequency energyin an interacting relationship with the slow mode of said beam, meansfor transferring slow mode noise energy to the fast mode of said beamcomprising means for transmitting pump `frequency energy in couplingrelationship with said beam, the coupling of said pump energy with saidbeam giving rise to a coupled pump wave that travels at a velocitysubstantially equal to one-half the sum of the velocities of anuncoupled slow mode beam wave of the signal frequency and an uncoupledfast mode beam wave of a frequency equal to the difference of said pumpand signal frequencies.

11. The traveling wave tube of claim l0 wherein said beam ischaracterized by a first reduced plasma wavelength at said signalyfrequency and a second reduced plasma wavelength at a frequency equalto the difference of said pump frequency and said signal frequency, saidpump energy lbeing propagated in coupling relationship C) with the beamalong a distance L which is substantially defined by:

VT L M where )q is said lirst reduced plasma wavelength, )t2 is saidsecond reduced plasma wavelength and M is the fraction of beam currentmodulation by said pump energy.

12. The traveling wave tube of claim ,11 further comprising fast modenoise stripping apparatus interposed between said electron gun and saidpump energy propagating means.

References Cited in the tile of this patent UNITED STATES PATENTS2,494,721 Robertson Jan. 17, 1950 2,565,708 Warnecke et al. Aug. 28,1951 2,579,480 Feenberg Dec. 25, 1951 2,758,246 Norton Aug. 7, 19562,767,259 Peter Oct. 16, 1956 2,832,0011 Adler Apr. 22, 1958 2,958,001Ashkin et al. Oct. 25, 1960 2,972,702 Kompfner et al. Feb. 2'1, 196112,974,252 Quate Mar. 7, 1961 3,009,078 Ashkin Nov. 14, 1961

